You know, if this was the father posting that his DD1 (OP’s DSD) had these anger management issues and was being violent towards his dd2 (OP’s dd) and had other severe anger and violent outbursts as well as other behavioural problems he would get nothing but support on here and would be told that something needed to give and that potentially he and the child’s mother needed to seek professional help for their DD.
In fact if the OP was the DD’s biological mother and was posting the same she too would be told that. But because this is a stepmother things are different? Why is that?
If the OP’s dd was being treated in the same way by a classmate the advice would be to speak to the school about how they could protect her dd. Being a stepmother makes no difference in this case. The child spends weekends in the OP’s house. The OP’s dd is her sister, and it is no more acceptable for a child to slap her sister and destroy her possessions than it would be for her to slap her half sister or even if it was a classmate or friend.
OP, it seems very clear that You have done everything in your power for this child to be included in your household and to be a sister to your DD. It also seems clear though that the child has some very significant behavioural problems which need addressing, and which the parents seriously need to look at. And tbh while I think that there is absolutely some room for criticism of your DH, I also wonder whether he is possibly in denial because he knows there are issues but addressing them potentially means professionals being involved and “labels” being applied in terms of a potential diagnosis.
Behaving in the way she does is not normal at seven. But it doesn’t sound as if this is a sad scared little girl, it sounds like a child with some issues which aren’t being addressed.
Many years ago I had a friend whose child exhibited some very serious behaviours. He was violent, he was rigid in his thinking, he had other obsessions but obviously I’m not comparing him to the OP’s DSD. What I would say though is that he was excluded from school on numerous occasions, and when friend came to talk to me I advised her to get some professional help for him because his behaviour wasn’t “normal” either. And her response was “but I don’t want him to be labelled.” To which I replied “but he’s already been labelled. He’s been labelled as violent, as troublesome, as difficult, if you seek some professional advice for him he may still be labelled, but it will be a label which means his difficulties will be recognised and he can be helped to deal with them. The alternative is that in ten years time this won’t be his classmates he’s beating up, it will be his partner.”
She went to the GP and two years on he was diagnosed on the autistic spectrum.
It’s entirely possible that DSD has some issues which mean that she will need a “label” in order to help her to deal with those issues, because to not deal with them is going to be far worse for her in the long run.
Is it possible to have that conversation with your DH along those lines OP?
And FWIW, I don’t think you have done anything wrong. And neither do I think that it’s wrong to tell a child how their behaviour makes you feel, even if you’re not their biological parent.